Plastic package for an integrated electronic semiconductor device

ABSTRACT

The invention relates to a plastic package for an integrated electronic semiconductor device to be encapsulated within a plastic body, the plastic bodyes formed using the step of molding said plastic body so as to fully enclose a semiconductor element, on which an integrated electronic circuit has been formed and which is placed onto a metal leadframe connected electrically to said integrated electronic circuit and carrying a plurality of terminal leads for external electric connection. To compensate the outward bends uncontrollably undergone by the plastic body due to thermal stresses during the molding step, a mold is used which has a cavity delimited by perimeter walls which define a concave-shaped volume. Preferably, at least one of the large walls, a bottom wall and a top wall, has a curvature inwardly of said mold. cavity. The curvature values are predetermined to compensate any outward curvature undergone by corresponding surfaces of said plastic body during the molding step. The plastic package thus obtained exhibits, at the end of the molding step, a body having a diversified thickness which is at a maximum near the edges and at a minimum in the central portion; the difference between these thicknesses is proportional to the amount of the relative deformation undergone by the central regions of the body surfaces with respect to the regions near the edges. The plastic body is within ideal overall dimensions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved method for forming plasticpackages for electronic semiconductor devices, which is particularlyuseful in the instance of thin packages including a supporting andelectric connection metal frame and a plastic body. The invention alsorelates to a package with improved characteristics. The invention isspecifically directed to the use of a mold of improved shape for formingthe plastic casing.

2. Description of the Prior Art

As is well known, integrated electronic semiconductor devices,comprising an integrated circuit, are formed on go-called “dies” of asemiconductor material which are mounted integrally to electricinterconnection structures enclosed in a body of a synthetic plasticmaterial serving a protective function.

In the specific field of this invention, such structures typicallycomprise a support, usually a thin metal foil (leadframe), the centralportion whereof, usually depressed, accommodates the die, and which hasterminal leads. The die is, across one of its large surfaces, in contactwith the leadframe, the integrated circuit being exposed on the oppositedie surface in electric contact with the leads. A plastic body fullyencloses the integrated device, and in part the leadframe on which thisis mounted. The terminal parts of the leads, or pins, are located on thebody exterior and function as electric connections for connecting thedevice to an external circuit.

Within the scope of this invention, of special concern arereduced-thickness packages, having body length and width which are muchgreater than its thickness. These are commonly referred to as thinpackages, and are supplied to the field in different types and fordifferent applications. Usually, they are employed for devices whichrequire no large power loss, that is, devices which are designed foroperating on comparatively small currents. For a better understanding ofthe aspects of this invention, it pays to briefly review the usual stepsof a conventional package forming process.

Typically, the dies are first placed on a certain number of leadframesformed on a single metal strip, and the electric connections to theleads are established using thin metal wires. The strip carrying thedies is then introduced into a mold having cavities corresponding to theindividual devices, and a molten, electrically insulating material isinjected at a high temperature to form the monolithic plastic body ofthe package. This material is usually a synthetic resin, such as anepoxy resin. The molding step includes a number of stages at which thetemperature is gradually changed to avoid any risks of breaking thesemiconductor material or, in any case, reducing the device reliability.On this account, moreover, the leadframe is heated before the moltenmaterial is poured in. After injecting the resin, that is after themolding in the proper sense of the word, and following a first coolingstep with consequent partial polymerization, the resin is subjected tothermodynamic curing processes. The latter will improve the materialproperties by promoting thorough polymerization and stretching the longmolecules which compose the polymeric material of the resin. The batchof packages thus formed is then taken out of the mold.

A known mold for injecting the resin is depicted in FIG. 1, which showsan exploded vertical section view taken through a single cavity, i.e. asingle package, of the batch. A leadframe 1 having a semiconductor diemounted thereon is placed into the mold cavity with the ends of theterminal leads protruding outside the cavity. The mold comprises, in theembodiment shown, two parts: a lower half-mold or half-shell die 3 andan upper half-mold or half-shell 4, each having a corresponding hollowtherein. The two half molds are disposed with their hollows facing eachother to define a cavity wherein to the resin is then injected throughan inlet port 5 formed in the mold itself.

In accordance with the prior art, the large parametric walls delimitingthe cavity (bottom wall 6 and top wall 7) are made flat in order toideally produce a body having flat parallel top and bottom surfaces.Actually, despite the precautions taken to avoid exposure of the wholedevice to abrupt temperature changes brought about by the aforementionedthermal treatments, the package leaves the molding and drawing stepswith a bending or so-called warpage deformation in it. The largesurfaces of the resultant body are not flat, but with the central regionof each surface that is sunk below or raised above the plane describedby the corresponding edges.

This behavior is due to the fact that the different materials which makeup the complete structure of the package, i.e. the metal leadframe, thesemiconductor die, and the resin, have different thermal properties(different expansion and shrinkage coefficients). In addition, theasymmetrical distribution about the horizontal axis A—A, as shown inFIG. 1, of the materials, i.e. the leadframe 1 and the semiconductor die2, inside the package body, generates internal stresses which contributetoward a warped package.

In other words, the ideal plane containing the leadframe—represented insection by the axis A—A—is subjected to compression in one of thedirections shown by the arrows C1 and C2 in FIG. 1. This compressioncauses a deflection to occur in said plane, and hence in the body as awhole. The actual shape of the package at room temperature is that shownin FIGS. 2 and 3, which are sectional views taken along either of thelarge dimensions of the package (vertical sections). The amount of thedeformation undergone has been exaggerated in the drawing figures forclarity. Viewed in cross-section, the body outline is basically that ofa parallelogram with two long sides showing a concavity in the samedirection.

Shown in FIG. 2 is a package with a concavity upwards, whereas in FIG. 3the package has a concavity downwards. The direction of the bending isindividual to each package and not fully predictable, although adeformation of the kind shown in FIG. 2 is more likely to occur with aninternal structure of the package having the distribution of thematerials shown in the figures.

As shown in both FIGS. 2 and 3, the device as a whole, i.e. both theleadframe 1 (together with the semiconductor die 2 mounted on it) andthe plastic body, generally denoted by 8, is bent at this intermediatestage of its forming process. In particular, the body departs from theideal shape, with flat top and bottom surfaces, by a degree ofdepression or elevation, with respect to a plane described by the edgesof each of these surfaces, of a plane tangent the corresponding surfacein its central region.

This depression (or elevation) is known as warpage, designated W inFIGS. 2 and 3. With a body illustratively measuring about 18-20 mm inwidth and length and about 1 mm in thickness, as would be typical ofcurrent thin packages, warpage may amount to a few tens of microns.Although the amount of warpage deformation is proportionally smallcompared to the size of the body, still it can grow into a seriousproblem during the following steps for completing the forming of thedevice, and does lower the reliability of the finished device.

After removing the packages from the mold, according to the prior art,the individual packages are separated from one another (the so-calledsingulation step) by cutting up the metal strip on which the batch ofleadframes has been formed. Thereafter, for each package, that portionof the leads which has been allowed to protrude out of the package bodyis bent to create pins (the so-called lead forming step) that willenable the electric connection of the device for a particularapplication, usually by welding the pins to a printed circuit.

During both of these operations, and as shown schematically in FIG. 4,the plastic body 8 of the package is held between a bottom clampingsurface 9 and a top clamping surface 10. Also, its position is set suchthat it is centered between side retaining means, not shown in FIG. 4.The effective clamping distance H, in practice the gap between the twoclamping surfaces 9 and 10, must be preset in the equipment used forthese manufacturing steps. To take account of the actual shape of thebody, and ensure proper clamping thereof, the distance H is set for anaverage amount of bending deformation in a package. This is calculatedas the sum of the ideal (i.e., undeformed) height of the body—asdetermined by the thickness of the Mold cavity—plus the averagedeformation thereof. Indicated by arrows in FIG. 4 are the forces Fwhich are applied to the package, specifically at the points of contactbetween the plastic body 8 and the clamping surfaces 9 and 10.

Despite the equipment having been preset, problems may be encounteredduring these final steps of the device forming process. In the instanceof packages which exit the mold in a markedly bent state, or, at anyrate, with an above-average amount of curvature, the unevenlydistributed stresses to which the body would be subjected (as shown inFIG. 4 by the arrows F) are quite large, and the package would receivean appreciable bending moment.

Accordingly, there exists a risk of straining the structure of thepackage or, in the extreme, of breaking the semiconductor die orstarting a delamination between internal parts of the body, i.e. of thesemiconductor die parting from the leadframe. On the other hand, aslightly deformed—less than a predetermined average amount—packagecannot be held securely between the clamping surfaces nor correctlycentered.

The likely outcome of this are processing difficulties and/or poorresults, which would result in an increased number of rejections. Asregards the structural and functional characteristics of the finisheddevice, and hence its reliability, a heavy warpage deformation canresult in failure to meet the specifications for a particularapplication of the device. Major factors in the context of thisinvention are the overall height of the device (as measured to theoutside of pine and plastic body), and the so-called standoff, that isthe relative height of the lowermost point on the body and of a planedescribed by the pin base.

FIGS. 5 and 6 show two prior art devices upon completion of theirforming process. The device of FIG. 5 has been obtained by bending thepins 1 a and 1 b to opposite directions with respect to the bendingundergone by the body 8. In the ideal instance of a body developing nodeformation, the device standoff could be exactly equal to the constantrelative height of the flat bottom surface of the body above a restplane described by the pin base.

The introduction of warpage diminishes, as shown in FIG. 5, the actualstandoff value, denoted by S, with respect to the ideal value, denotedby S_(ideale). In fact, the central region of the bottom surface 11 ofthe plastic body 8 is sunk with respect to the edges, due to the bendingdeformation. FIG. 5 illustrates an extreme case where this centralregion is also sunk with respect to the pin base. In all cases, the pinsmay fail to adhere tightly to the surface on which the package rests. Atypical amount of standoff would be in the range of 0.1 (0.05 mm. Thus,the admissible error may be approximately the same magnitude as thewarpage deformation.

The amount of standoff may prove inadequate to ensure electric contactof the device with a printed circuit upon welding, or in any case, maybe less than a minimum provided for by its specifications.

FIG. 6 shows a package wherein the pins have been bent in the samedirection as the curvature of the body. Here, the effect of the bodywarpage deformation is that of increasing the overall height of thedevice. Since in this instance the top surface 12 of the body 8 is bent,with a raised central region, the overall height of the device, asdenoted by T in FIG. 6, is given by the ideal overall height of thedevice, denoted by T_(ideale) in the same Figure, plus the amount ofwarpage W.

As shown, the ideal overall height T_(ideale) can be obtained as the sumof the constant ideal thickness B of the plastic body 8 plus the amountof standoff S_(ideale), which in this case can be considered to besubstantially the same as the actual standoff. Where the effect of thedeformation is particularly evident, the packages may deviate from thespecifications set for the device application, thereby increasing therate of production rejections.

SUMMARY OF THE INVENTION

A first object of this invention is to provide a package for integratedcircuits which has improved structural features. A closely relatedobject is to reduce the number of rejections at the reliability testingstage. A first particular object is to make the amount of standoff inthe finished device independent of the bending deformation in theplastic body.

A second particular object is to make the device overall height valuecontrollable and repeatable. A further object of this invention is toimprove those functional characteristics of the package which canenhance its reliability, by improving control of the problems whicharise during the device forming process. Particularly during the processsteps which follow the body molding, this invention is aimed atimproving the plastic body clamping and, therefore, its centering, withconsequent improved accuracy of its manufacturing process.

The invention is also aimed at reducing the risk of breaking thesemiconductor die, or of delamination problems inside the body. Arelated object is to provide devices wherein the vertical overalldimension of the body is repeatable and within the ideal thicknessvalue.

Still another object of this invention is to provide a simple processfor forming the body which can yield a package of controllable shape andconforming with the specifications set for its application.

The solution which this invention incorporates is that the deformationthat the body is bound to undergo, at least in part in an uncontrollableway due to thermal stressing during its formation, specifically duringthe molding step, can be compensated for by making preliminary changesto the shape of the plastic body predetermined for the molding. Inparticular, outward bendings which would result in the body exceedingits ideal overall dimensions, must be compensated for. In practice, amold is used whose cavity shape deviates from the ideal by a deformationwhich is equal to and oppositely directed from the maximum deformationanticipated for a body, so that a deformed body is released from themolding step which extends in no directions beyond the shape of an idealbody.

The features and advantages of a device obtained by this invention willbe more clearly apparent from the description of an embodiment thereof,given by way of non-limitative example with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded sectional view of a prior art mold for forming aplastic package for an integrated circuit, shown during the body moldingstep.

FIGS. 2 and 3 are sectional views of a prior art plastic package, shownupon exiting the mold and having concavities upwards and downwards,respectively.

FIG. 4 is a sectional view showing the forces which act on a plasticpackage of conventional construction during the device formation stepswhich follow the molding step.

FIGS. 5 and 6 are sectional views of a finished prior art device withits pins respectively bent to the same direction as and the oppositedirection from the bending undergone by the body.

FIG. 7 is a sectional view of a mold for forming a plastic package,according to the invention.

FIG. 8 is an exploded sectional view of a mold for forming a plasticpackage for an integrated circuit, according to this invention, shown atthe body molding step.

FIG. 9 is a sectional view illustrating the step of injecting theplastic material into the mold, in accordance with this invention.

FIGS. 10 and 11 are sectional views showing a plastic package as itexits the mold with concavities upwards and downwards, respectively, inaccordance with this invention.

FIGS. 12 and 13 are sectional views of a finished device according tothe invention, having its pins respectively bent to the same directionas and the opposite direction from the bending undergone by the body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 7-13, to be described herein below, are sectional views, not drawnto scale, taken along either of the large dimensions of the package. Thecurvatures of the perimeter walls delimiting the mold cavity for formingthe plastic body and the deformations undergone by the device have beenexaggerated for clarity of illustration. This description will cover oneembodiment of this invention.

With reference in particular to FIG. 7, a mold 13 for injecting anelectrically insulating material is depicted. Only one cavity of themold for forming a single device will be described. Actually, a numberof such devices would be formed simultaneously, as previously describedin connection with the prior art.

In a preferred embodiment of this invention, the mold is conventionallymade up of two superimposable half-shells, or half-molds, namely a lowerhalf-mold 14 and upper half-mold 15, each with a corresponding hollow.The two half-molds 14 and 15 are superposed and delimit a cavity ontheir interiors which is communicated to an inlet port 5 for the moltenmaterial. This inlet port 5 is shown to be formed in the upper half-mold15, similar to the example described in connection with the prior art.In a practical embodiment, the inlet port 5 could be formed in the lowerhalf-mold 14 or in both half-molds, 14 and 15.

According to this invention, the large delimiting walls of the cavity,that is the bottom wall (inside the lower half-mold 14) and the top wall(inside the upper half-mold 15) denoted by 16 and 17, respectively, arenot flat. They are rather given a curvature, w with concavities facingin opposite directions, respectively upwards and downwards. Thus, if adevice could be ideally produced which underwent no deformation duringits formation steps, then the large outward surfaces of the plastic bodywould be bent inwardly.

In either case, and regardless of the deformation actually undergone bythe package, the thickness of the resultant body is unevenlydistributed, being smaller in the central region than in the regionsnear the edges. The amount of the curvature, preset for the walls 16 and17 delimiting the cavity, is selected to compensate for the effect ofwarpage, already discussed in connection with the prior art, on the bodyand especially on the surface being bent outwardly.

That is, the amount of said curvature is equal to the maximum curvatureanticipated for the body under deforming stress in one direction or theopposite one (concavity upwards or downwards in FIGS. 2 and 3). Whatwill be delivered from the inventive mold is, therefore, a plastic bodyhaving a predictable vertical overall dimension which is unaffected byany deformation undergone by an individual package, this verticaloverall dimension being equal to the distance between the mol d surfaces16 and 17 along g the edges. This v vertical overall dimension isdenoted by I in FIG. 7.

The absolute value of the curvature is, in the embodiment shown,approximately the same for both walls 16 and 17, such beingapproximately also the effect of the thermal bending deformation on thecorresponding surfaces of the formed body. In FIG. 7, the elevation anddepression, respectively, in the central region of the bottom wall 16and the top wall 17 with respect to a plane described by the edges ofthe corresponding surface is denoted by Was. This is the maximum amountof warpage S to which the body is subjected.

A typical preferred value for Wag is 40 μm (micrometers) for a plasticmaterial having a shrinkage coefficient of 17 E-6/° C. and for packagesof the TSOP (Thin Small Outline Package) type, measuring 18-20 mm inlength and width and having an average thickness of 1 mm or less, inaccordance with the latest developments in the art. Statistical analysisprocedures are used to determine the maximum warpage W_(MAX). Themanufacturing steps for a device with a plastic body, according to thisinvention, and the device itself as it appears at different stages ofits formation, are illustrated by FIGS. 8-13.

Throughout these Figures, and similar to the foregoing considerationsmade about the prior art, the numeral 1 denotes a metal leadframe,usually a worked foil provided with terminal leads and having asemiconductor die 2 mounted thereon, usually in a sunk central region ofthe leadframe, which has been formed with an integrated circuit. Theintegrated circuit, as usual in the art, is in electric contact with theleadframe, or more precisely, electrically connected to the leads bymeans of thin metal wires.

As shown in FIG. 8, the leadframe 1 is placed between the two half-molds14 and 15. Specifically, the leadframe 1 and the die 2 mounted thereonare placed into the hollow formed in the lower half-mold 14, and thelead ends are laid onto the half-mold 14 outside the hollow. Theintegrated circuit 2, in particular, is centered in the hollow. Theupper half-mold 15 is then superimposed, with its hollow aligned to matewith the hollow formed in the lower half-mold 14, thereby defining acavity between the two half-shells 14 and 15. After clamping the twohalf-molds 14 and 15, the plastic body forming operation is started.

As shown in FIG. 9, a plastic molten material at a high temperature,e.g. an epoxy resin, is injected under pressure into the cavity soobtained through specially arranged channels, not shown, and the inletport 5. This is followed by different thermodynamic cooling and resincuring processes, already described in connection with the prior art.During the steps just discussed, due to thermal stressing, the bodygenerally undergoes a bending deformation or warpage, similar to aconventional plastic package.

According to this invention, however, the warpage effect on the shape ofthe body actually produced is fully compensated for by the differentshape of the mold cavity employed. The warpage yields a body which hasone of its large outward surfaces approximately flat and the othersurface with a double curvature compared to that produced by warpage ina standard package.

In particular, FIGS. 10 and 11 are sectional views showing schematicallya package at room temperature, as it appears after thorough cooling uponexiting the mold.

Shown in FIG. 10 is a package which underwent deformation similar toFIG. 2, that is with a concavity upwards. Schematically shown is, inparticular, a package which has been warped to a maximum extent. As canbe seen, the bottom surface 18 of the package is flat because thewarping action produces an oppositely directed curvature with respect tothe compensating curvature built in the corresponding delimiting wall ofthe mold cavity (wall 16 in FIGS. 7-8), and is fully compensated for.

The top surface 19 has instead a double inward curvature because thebending due to warpage adds to that imparted by the mold. Of course, inthose packages which have been warped to less than the maximum valuethat has been taken into account in setting the mold wall surfaces, thebottom surface 18 will not be truly flat, and will bend more or lessinward of the body. On the other hand, the top surface 19 may develop asmaller curvature than the maximum shown in FIG. 10.

FIG. 11 illustrates instead the behavior of a package which, accordingto the prior art, would develop its maximum concavity downwards. In thiscase, the substantially flat outward surface is the upper surface 20,while the bottom surface 21 has twice the curvature of a conventionalpackage. The fact that the device according to the invention may showtwice the amount of curvature on an outward surface thereof representsno problem for the following step, but rather avoids the problemsdiscussed in connection with the current technique.

The bending of all the large surfaces of the body in the inventivedevice always occurs, in fact, along the inward direction of the body.It can be seen in both FIGS. 10 and 11 that, whatever the amount ofwarpage developed, the body will never exceed, either downwardly orupwardly, a vertical overall dimension denoted by I which corresponds tothe greatest thickness of the body near its edges. This verticaldimension can be preset from the dimensions of the mold cavity employed,and is in particular equal to the distance (distance I in FIG. 7)between the wall surfaces 16 and 17 of the mold at the edges.

The distance I may be used to advantage for setting the equipment usedin the subsequent forming steps of the device. In the subsequentcompletion steps of the device forming operation, according to the priorart described hereinabove and schematically shown in FIG. 4 by way ofexample, the package is placed in equipment having package clampingsurfaces for separating the individual devices and bending the pins.

The distance H between the clamping surfaces, to be set before thepackages are introduced, for a package according to the invention, asshown in FIGS. 10 and 11 by way of example, is related non-critically tothe bending deformation of the individual package. This distance can beadvantageously selected to be equal to the vertical overall dimension I.The edges of both the bottom and top surfaces (18 and 19 in FIG. 10; 21and 20 in FIG. 11) of the body constitute contact points regardless ofthe amount of warpage deformation. And after all, there are no othercentral points of contact. Thus, the package will be properly clampedbetween these points.

In this way, problems from an undefined setting value are avoided withthe equipment, and the risk is avoided of straining the structure, whilethe package alignment is improved.

This solution according to the invention offers, therefore, theadvantages of facilitating the following manufacturing steps andimproving the characteristics of the device.

FIGS. 12 and 13 show two typical devices at the end of the manufacturingprocess according to the inventions for two different forming methods.In both cases, the standoff and overall height of the device arecontrollable quantities.

In FIG. 12, the pins 1 a and 1 b have been bent to the oppositedirection from the bending of the body. In particular, the most commonresult of a standard formation, that is with the die 2 in an upwardposition relative to its leadframe 1, is shown for a package which, onexiting the mold, has its concavity upwards as shown in FIG. 10.

Of course, the same considerations would apply to a reversely formedpackage, that is with the die 2 under the leadframe 1, and to a bodyhaving its concavity downwards as in FIG. 11. As can be seen, inasmuchas the bottom surface 18 of the plastic body is flat or at least notdownward bent, the central region is no longer sunk from a planedescribed by the edges. This means that the minimum standoff isconstant, equal to the ideal S_(ideale), and hence unrelated to thedeformation.

The standoff can be selected as an operational parameter included to theapplicational specifications of the device. A device with pins (1 a and1 b) bent to the same direction as the curvature undergone by the bodyis shown in section in FIG. 13, specifically for reverse forming withthe concavity upwards.

In the same way as described in relation to FIG. 12, this would beequivalent to considering a package formed in a standard way but showinga concavity downwards as released from the mold. In this case, theparameter which caused problems to prior art devices was the overallheight of the device T. Here, as can be gathered from FIG. 13, inasmuchas the top surface 20 is flat, the central region of the surface is nolonger raised above a plane described by the edges. The overall heightis, therefore, equal to the ideal T_(ideale), and can be obtained as thesum of the overall dimension of the plastic body (not specified in FIG.13) plus the standoff S_(ideale), and is hence unaffected by warpagedeformation.

This invention has the advantage of providing packages with improvedreliability. It provides a smaller number of packages out ofspecifications and accordingly, a reduced statistical number ofrejections during testing of the finished device. The solution proposedallows the negative effects of warpage deformation on the devicecharacteristics to be controlled and removed.

In particular, it is now possible to control the values of the standoffand the overall height so as to keep them within specifications. Theoptimized standoff has the advantage of yielding devices which haveimproved electric contact (better welding) with the package restsurface.

The solution provided by this invention has a further advantage in thatit makes for improved repeatability of the device forming processesthrough the control of the package characteristics. This leads todecreased rejections from production and, hence, to savings in costs.

The manufacturing steps subsequent to the formation of the body,according to this invention, are facilitated, particularly becausepackage clamping and centering problems are eliminated by thepossibility of accurately determining the maximum overall dimension ofthe body. Greatly reduced is also the risk of breaking the package frommechanical straining, inasmuch as the body has no surfaces bent alongthe outward direction.

In addition, this invention allows a package to be formed with improvedcharacteristics using a simple process. While only one embodiment ofthis invention has been described and illustrated, many modificationsand variations are possible consistent with the basic concept of theinvention as set forth in the appended claims. For example, it is notnecessary for the mold to be comprised of upper and lower half-molds,but there could be a different mold construction. Correspondinglytherewith, some other forming technologies could be used. The delimitingwalls of the mold cavity could show some irregularities, while stillproviding an envelope substantially in the form of a spherical segment.It would also be possible to provide a mold wherein only one of thelarge delimiting walls of the cavity is given a curvature.

Furthermore, the overall shape of the package, illustrated herein inschematic form, encompasses different types of packages, and changes maybe made thereunto within the scope of the present inventive idea.

I claim:
 1. A plastic package for an integrated electronic semiconductordevice, comprising: a metal leadframe on which a semniconductor elementis placed, wherein an integrated electronic circuit has been formed onsaid semiconductor element and is electrically connected to said metalleadframe; and a plastic body which encloses said semiconductor elementand said leadframe so as to leave outside, for electrical connection,ends of a plurality of terminal leads formed on said metal leadframe;wherein said plastic body has physical characteristics of a plastic bodyformed by a molding process, has a maximum thickness near the edges andhas a minimum thickness in the central portion, wherein the differencebetween said maximum and minimum thickness is twice a maximum expecteddeformation of the package during the molding step.
 2. A plastic packageaccording to claim 1, wherein the large surfaces of said plastic body, abottom surface and a top surface, both have a curved shape inwardly ofthe plastic body.
 3. A plastic package according to claim 1, wherein theratio between length and average thickness of the plastic body is in arange of approximately
 20. 4. A plastic package according to claim 1,wherein one of the large surfaces of said plastic body, a bottom surfaceand a top surface, is flat while the other surface has a curved shape.5. A plastic package for an integrated electronic semiconductor devicecomprising: a metal leadframe on which a semiconductor element isplaced, wherein an integrated electronic circuit has been formed on thesemiconductor element and is electrically connected to the metalleadframe; and a plastic body enclosing the semiconductor elements andthe leadframe so as to leave outside, for electrical connection, ends ofplurality of terminal leads formed on the metal leadframe, wherein theplastic body has two surfaces, a top surface and a bottom surface,wherein one of the two surfaces has a concave shape, while the othersurface is substantially flat such that the plastic body has a maximumthickness near the edges and a minimum thickness in a central portion ofthe body, wherein a difference between the maximum thickness and theminimum thickness is twice a maximum expected deformation of the plasticpackage during formation of the plastic package by a molding process. 6.The plastic package according to claim 5, wherein a difference betweenthe maximum thickness and the minimum thickness is twice a maximumexpected deformation of the plastic package during formation of theplastic package by a molding process.
 7. The plastic package of claim 5,wherein the ratio between length and average thickness of the plasticbody is in a range of approximately 20.